Answer
400 grit is for fine prep and refinement after 320 and before 600.
Why it happens
400 grit is a fine grit for surface refinement, primer prep, and light smoothing.
Recommended grit
Use 400 after 320 or before 600 when a finer prep is needed.
Wet or dry
Dry for fine prep; wet for fine finishing stages.
Success check
The surface is refined without deep scratches.
What to do
- Use 400 after the surface is already level.
- Sand lightly.
- Clean before moving to 600 or finishing.
- Check coating instructions for final prep requirements.
Avoid: Do not use 400 for heavy removal.